1. Field of the Invention
The present invention relates to a printer for performing electrophotographic image printing on a recording paper. The present invention also relates to a transfer unit to be incorporated in such a printer.
2. Description of the Related Art
Conventionally, an electrophotographic printer is provided with a photosensitive member having a surface for forming an electrostatic latent image, a developer unit for developing the electrostatic latent image into a toner image, a transfer unit for transferring the toner image onto a recording paper, and a fixing unit for fixing the toner image on the recording paper. FIG. 6 illustrates an example of prior art transfer unit incorporated in such a printer.
The transfer unit Ue shown in FIG. 6 is arranged in facing relationship to a surface 90a of a photosensitive drum 90. The transfer unit Ue includes a transfer wire 93, a discharge wire 94, a first guide 91 and a second guide 92. A transfer current flows through the transfer wire 93 for transferring a toner image formed on the surface 90a of the photosensitive drum 90 to a recording paper P. For example, while the toner image formed on the photosensitive drum 90 is negatively charged, the transfer current passing through the transfer wire 93 positively charges the recording paper P for attracting the toner to the recording paper P. On the other hand, a discharge current flows through the discharge wire 94 for releasing the electric charge on the recording paper P after the image transfer. The discharging operation by the discharge wire 94 prevents the recording paper P from adhering to various parts of the printer in the subsequent paper transport path. The first and the second guides 91, 92, functioning to guide the recording paper P, may be made of an wear-resistant metal for avoiding early wearing due to contact with the recording paper P. The first and the second guides 91, 92 are spaced from each other in a recording paper transport direction so as not to shield the transfer wire 93 and the discharge wire 94 relative to the photosensitive drum. Specifically, the first guide 91 is arranged upstream from the transfer wire 93 in the recording paper transport direction, whereas the second guide 92 is arranged downstream from the discharge wire 94 in the recording paper transport direction.
In the printer including the above-described transfer unit Ue, the length La of the paper P (the length in the paper transport direction) winding around the photosensitive drum 90 should be as small as possible to obtain a high quality printed image for the following reasons. Since the recording paper P is transported by a transport mechanism (not shown) provided separately from a rotary mechanism (not shown) of the photosensitive drum 90, there exists a slight difference between the transport speed v1 of the recording paper P and the peripheral speed v2 at the surface 90a of the photosensitive drum 90. Therefore, when the winding length La is large, the recording paper P may become loose, or the drum 90 may rub against the paper P. This causes an improper toner image transfer to the recording paper P, resulting in a degradation of the printed image quality.
However, in the prior art transfer unit Ue, the first and the second guides 91, 92 are arranged at front positions which entirely avoid the transfer wire 93 and the discharge wire 94. Therefore, the distance Lb between the first and the second guides 91, 92 is relatively large, which inevitably increases the winding length La of the recording paper P about the photosensitive drum 90. As described above, such a large winding length La causes a deteriorated printed image.
It is therefore an object of the present invention to provide a printer capable of providing a high quality printed image by reducing the winding length of a recording paper about a photosensitive member.
Another object of the present invention is to provide a transfer unit which may be advantageously incorporated in such a printer.
In accordance with a first aspect of the present invention, there is provided a printer comprises a photosensitive member having a surface for forming a toner image; a transfer member for passing a transfer current for transferring the toner image onto a recording paper, the transfer member being arranged to face said surface of the photosensitive member; a discharge member for passing a discharge current for performing discharge of the recording paper, the discharge member being arranged downstream from the transfer member in a recording paper transport direction to face said surface of the photosensitive member; a first guide arranged upstream from the transfer member in the recording paper transport direction for guiding the recording paper; and a second guide arranged downstream from the first guide in the recording paper transport direction for guiding the recording paper. The second guide includes a contact portion located between the transfer member and the discharge member in the recording paper transport direction for contact with the recording paper, and the contact portion is positioned adjacent an unshielding portion for exposing the discharge member relative to said surface of the photosensitive member.
The technical advantages of the printer having the above-described structure will be specifically described later with reference to preferred embodiments of the present invention.
In one embodiment, the second guide extends from a first position located between the transfer member and the discharge member in the recording paper transport direction to a second position located downstream from the discharge member in the recording paper transport direction, and an edge of the second guide located at the first position serves as the contact portion. In this embodiment, the unshielding portion may be provided by a single slit formed in the second guide along and adjacent said edge of the second guide. Alternatively, the unshielding portion may be provided by a row of slits formed in the second guide along and adjacent said edge of the second guide.
In another embodiment, the transfer member and the discharge member are separated from each other by a partition wall which also serves as the second guide, and the contact portion is provided by a forward end of the partition wall. In this embodiment, the forward end of the partition wall serving as the contact portion may be bent downstream in the recording paper transport direction.
According to a second aspect of the present invention, there is provided a transfer unit to be brought into contact with a recording paper, comprising a transfer member arranged to face the recording paper for passing a transfer current; a discharge member for passing a discharge current for performing discharge of the recording paper, the discharge member being arranged to face the recording paper downstream from the transfer member in a recording paper transport direction; a first guide arranged upstream from the transfer member in the recording paper transport direction for guiding the recording paper; and a second guide arranged downstream from the first guide in the recording paper transport direction for guiding the recording paper. The second guide includes a contact portion located between the transfer member and the discharge member in the recording paper transport direction for contact with the recording paper, and the contact portion is positioned adjacent an unshielding portion for exposing the discharge member relative to the recording paper.
Other objects, features and advantages of the present invention will become clearer from the detailed description given below with reference to the accompanying drawings.